1. Technical Field
The present invention relates to a composition for forming a substrate, and a prepreg and substrate using the same. More particularly, the present invention relates to a composition for forming a substrate, comprising a liquid crystal thermosetting oligomer having one or more soluble structural units in the main chain thereof and having thermosetting groups at one or more ends of its main chain, and a metal alkoxide compound having reaction groups which can be covalently bonded with the thermosetting groups, and to a prepreg and substrate using the same.
2. Description of the Related Art
With the advancement of electronic appliances, printed circuit boards (PCB) are becoming lighter, thinner and smaller day by day. In order to meet these requirements, wiring of a printed circuit board is also becoming more complicated and highly densified. For this reason, electrical stability, thermal stability and mechanical stability are important factors affecting a printed circuit board. In particular, among them, thermal stability, for example, the coefficient of thermal expansion (CTE) is one of the important factors influencing reliability at the time of manufacturing a printed circuit board.
A printed circuit board chiefly comprises copper serving as circuit wiring and a polymer serving as an interlayer insulator. The CTE of the polymer constituting an insulation layer is much higher than those of copper. In order to overcome the difference in CTE between the polymer and copper, the CTE of the polymer constituting an insulation layer is being decreased by impregnating the polymer into a non-woven glass fiber fabric or adding an inorganic filler to the polymer.
Generally, with the increase in the amount of the inorganic filler added to the polymer, the CTE of the polymer decreases, but there is a limit to using this technique in the process of manufacturing a printed circuit board. Further, in order to meet the requirement for highly-densified fine patterns, surface roughness is also considered as an important factor. The size of an inorganic filler added in order to control the surface roughness of the printed circuit board is becoming smaller. However, as the size of the inorganic filler decreases, the problem with the uniform dispersibility of the inorganic filler is on the rise, and, particularly, the problem that the nanoscale filler must be uniformly dispersed is also on the rise.
Ultimately, a polymer material having a coefficient of thermal expansion equal to that of copper is required. However, conventional polymer materials, which are obtained by adjusting the kind and content of a polymer and the size and content of an inorganic filler, cannot sufficiently satisfy such a requirement.
Generally, an insulation layer of a printed circuit board is chiefly made of epoxy. Epoxy itself has a CTE of about 70˜100 ppm/° C. In order to decrease the CTE thereof, epoxy is impregnated into a non-woven glass fiber fabric, or an inorganic filling having a low CTE is added to an epoxy matrix, thereby realizing epoxy having a low CTE (refer to FIG. 1). The CTE of epoxy is linearly decreased in proportion to the amount of added inorganic filer. However, when a large amount of inorganic filler is added, the CTE of epoxy decreases, but the viscosity of epoxy rapidly increases, so that it is difficult to form a product.
In particular, in the case of an insulation film having a multilayer structure which is used for a printed circuit board, it is impossible to perform interlayer coupling.
For these limitations, the CTE of epoxy itself is decreased, and simultaneously a critical amount of an inorganic filler is added thereto. In order to decrease the CTE of epoxy itself, epoxy resins having different structures from each other are mixed and then used. In this case, the component and composition of each of the epoxy resins play an important role in the decrease in the CTE thereof. Further, the CTE of epoxy is greatly influenced by the kind, size and shape of the added inorganic filler as well as the amount thereof. Although the size of the added inorganic filler must be decreased to a very large extent, that is, must be on the nanoscale in size in order to realize ultrafine patterns, it is difficult to obtain a film uniformly dispersed therein with the inorganic filler.
As such, it is limited to realizing thin and highly-densified integrated circuit patterns, and it is difficult to satisfy the required thermal, electrical and mechanical properties.